The invention relates to a hydrocarbon mooring and transfer system comprising a tower resting on the seabed, a vessel containing hydrocarbons, anchoring means comprising at least four spaced-apart anchoring members connected via a respective anchor line to the vessel, a hydrocarbon transfer duct extending between a coupling position, located between the bow and stern of the vessel, and the tower.
Such spread moored hydrocarbon transfer systems are known in the prior art, in particular for oil loading and or offloading to a tower.
Furthermore, from U.S. Pat. No. 4,826,354 an LNG pipeline system is known in which LNG is offloaded from a tanker to an offshore tower resting on the seabed and is transported to shore via a pipeline utilising expansion joints to compensate for contraction. Generally, LNG will be offloaded from the tanker to an onshore storage tank whereas vapour from the storage tank are recirculated to the vessel to keep the tanks under pressure. From the onshore storage, the LNG may be fed to a regasification plant and forwarded to the network.
The known mooring configurations for midship offloading have as a disadvantage that under specific directions of winds and currents, it will not be possible to load or offload at the tower. This could be compensated by providing a rotating transfer duct at the tower having a 360xc2x0 pipe swivel and dynamic positioning of the tanker vessel around the tower, which is however a costly solution.
An other option to moor an LNG carrier to an LNG offloading terminal comprising a loading crane is via a transfer connection at the stern of the vessel, anchorlines extending to several buoys from the bow and from the stern of the vessel in a spread moored configuration. The anchorlines at the bow of the vessel can be tensioned or slackened in such a way that the vessel can assume different positions depending on wind and current directions by fishtailing around the stem LNG transfer point. This system is described in Offshore Technology Conference 1825, 1973. In this mooring configuration, the transfer duct on the loading and offloading crane must follow the movements of the stern of the vessel, which are relatively large in view of the distance of the stern from the centre of the vessel and the limited fishtailing effect of a moored tanker.
It is therefore an object of the present invention to provide for a loading and offloading system, which can handle tankers having a midship loading and offloading facility.
It is a further object of the present invention to provide for a flexible mooring system in which the vessel can be moored in several orientations depending on wind and current, the transfer duct during loading and/or offloading remaining substantially in a stationary position.
It is again a further object of the present invention to provide for a relatively simple and cost effective mooring system which is particularly suitable for safe and stable loading and offloading of cryogenic fluids, such as LNG or LPG from a vessel to shore.
Hereto, the hydrocarbon mooring transfer system of the present invention is characterised in that the system comprises at least six anchoring members, at least four of which are connected to the vessel, the vessel being attachable to at least two different groups consisting of four anchoring members in at least two orientations.
By providing a number of anchoring points around the tower, each anchoring point comprising for instance one or more piles in combination with a polyester mooring line, one or more pivoting piles or a buoy, the tanker can be spread moored by attaching it to at least four buoys in different orientations, depending on wind and current directions, without it being necessary for the tower to have a fully weathervaning hydrocarbon transfer swivel. Such a selectively main orientation of the vessel can guarantee a stable positioning during the time in which the vessel is moored to the tower, which for an offloading operation may be for instance 20 hours for vessel position adjustment, connection of the mooring lines, connection of the hydrocarbon transfer arms, cooling down of the transfer duct, start-up of transfer, finishing and cleaning the transfer duct, disconnection of the transfer arms, mooring lines and takeoff off of the vessel.
Preferably, a number of eight mooring buoys are used, such that the vessel can be moored in four orientations forming an rectangular pattern around the tower, while loading or offloading from a midship position.
In a preferred embodiment, the vessel comprises a liquefied gas tanker, such as an LNG or LPG tanker. The offloaded LNG storage can be placed on shore but also offshore on a buoy including a regasification plant.
The vessel of the present invention can be a tanker vessel or a combined LNG storage and regasification plant. The vessel can be spread moored in a relatively stiff manner, but can also be provided with weathervaning capacities relative to the tower by employing a spread mooring system such as described in WO 01/10707 and WO 00/78600, in the name of the applicant.